1

CHANGES IN THE CONFIGURATION OF THE FRESH WATER LENS DUE TO

ASRRG, WESTERN PART OF THE TINY PARI ISLAND, SERIBU ISLANDS,

JAKARTA, INDONESIA

Edi Prasetyo Utomo

1, Ananta Purwoarminta

1

1Research Center for Geotechnology, Indonesian Institute of Sciences (LIPI)

Lipi Complex, Jl. Sangkuriang, Cisitu, Bandung 40135

Email: ediprasetyoutomo@gmail.com

ABSTRACT

Pari Island (Pulau Pari) is a small coral island in the Seribu Islands group off the north coast of

the Jakarta Capital Special Region. Government has planned to develop this island into a tourist

attraction because of its beautiful environment. However, the government development plans

have not included facilities to meet the demand for groundwater, which represents the principal

resource to meet the demand for freshwater. Pari Island has limited resources of groundwater due

to the small size of the area where recharge of groundwater from rainfall can take place. A

problem will arise that with the increase of visitors to the island the demand for freshwater will

increase, while groundwater resources of the island are very limited and dominated by brackish

and saline groundwater. The objective of this research was to explore the possibility to increase

the resources of fresh groundwater of Pari Island to support the increasing demand of fresh

water. The method employed in the research is the application of ASRRG technology by the

injection of the largest possible volume of rainwater and monitor changes of electrical

conductivity and geoelectrical resistivity. Installation of an ASRRG network proved effective in

lowering the salinity of groundwater in small islands. By increasing pore volume from 30% to

60% salinity decreases by 77%, which is reflected by the effect that the fresh groundwater lens

which before comprised brackish water became fresh water with a thickness of 2.5 meter. The

gain in pore volume equaled the decrease in salinity.

Key words: artificial recharge, small island hydrology, saltwater and freshwater relations, Pari

Island, Indonesia

ABSTRAK

Pulau pari merupakan pulau yang sangat kecil yang tersusun oleh batuan koral. Pulau ini

ditargetkan oleh pemerintah sebagai tujuan wisata karena keindahanya. Namun, target dari

pemerintah tersebut tidak diikuti dengan penyediaan fasilitas dalam pemenuhan kebutuhan

airtanah. Tanpa adanya eksploitasi airtanah, pulau pari mempunyai airtawar yang minim karena

sempitnya daerah imbuhan airtanah dari air hujan. Akibatnya muncul permasalahan dengan

meningkatnya kunjungan wisata, maka kebutuhan airbersih juga semakin meningkat. Sementara

airtanah tawar di pulau ini sangat terbatas dan lebih didominasi oleh air payau hingga asin.

Tujuan dari penelitian ini adalah untuk meningkatkan kapasitas airtanah tawar di pulau pari,

sehingga dapat menunjang kebutuhan airtawar. Metode yang digunakan dalam penelitian ini

adalah dengan pemasangan teknologi ASRRG dengan memasukkan air hujan sebanyak-

banyaknya yang selanjutnya dipantau melalui pengukuran DHL dan geolistrik. Pemasangan

ASRRG dengan system jaringan ini dinilai efektif dalam menurunkan kadar salinitas air tanah di

pulau kecil. Dengan menaikkan tingkat volume pori dari 30% menjadi 60% maka akan

menurunkan tingkat salinitas 77% dan dicermikan dengan ketebalan lensa airtanah tawar dari

2

yang sebelumnya payau menjadi airtawar dengan ketebalan 2,5 meter. Kenaikan volume pori

sebanding dengan penurunan salinitas.

Kata Kunci: imbuhan buatan, hidrologi pulau kecil, air asin dan air tawar, Pulau Pari, Indonesia

INTRODUCTION

As an archipelagic state, the policy of the Government of Indonesia to emphasize the

development of the maritime sector is a correct decision. The vast maritime resources, the coastal

resgions, and the small islands have not yet been adequately explored. Development of maritime

resources, coastal reareas, and small islands will open up new economic areas. The fishing

industry and tourism wil experience rapid development which in its turn could result in growing

social welfare. However this effort will face a number of limitations as coastal areas and small

islands primarily depend on limited fresh groundwater resources to meet the the demand for

fresh water. The limited fresh groundwater resources in the coastal areas and small islands is the

result of the interaction with seawater. On the other hand, human activity including industry and

tourism, will diminish the availability of fresh groundwater.

Pari Island is one of the Seribu Islands group of islands located in the Bay of Jakarta. The island

is classified as a very small island with an area of 41.32 Ha,[1]

with a length of 2.5 Km and

average width of 200 meters. The broadest part of the island is in the center of the islands, while

its width tapers off towards the ends of the island. Although a small island, around 800 people

live on the island. The provincial government of Jakarta aims to develop the island into a marine

tourism destination. Based on the data from the tourism management office on Pari Island, the

monthly number of tourists visiting the island is around 3000, peaking during holidays. This fact

gives rise to a new problem, which is that of clean water supply. Up till now meeting the demand

for freshwater for domestic purposes was met by groundwater, while bottled water brought from

Java Island served as drinking-water. Desalination technology has been employed on the island,

but with no success because the product was too expensive and could not be successfully

introduced and accepted by the local community.

However over-expoitation of groundwater could cause increasing salinity of the groundwater due

to seawater intrusion. Originally fresh groundwater could change to brackish and ever become

saltwater during the dry season. The occurrence and geometry of the groundwater body in small

islands is very much influenced by the geological conditions, tidal fluctuation and variation in

rainfall[2]

. In very small islands , fresh groundwater is replenished exclusively from precipitation

3

while significant groundwater uptake through evapotranspiration can occur in low lying and/or

vegetation covered areas [3,4,5,6,7]

.

GEOLOGY SETTING

The lithology of Pari Island can be observed in dug-wells and along the coastine (Fig. 1)

and consists of sand and coral. The sand layer varies in thickness between 0.5 – 4 meters, and is

underlain by coral. The sand interval is thicker at the central part of the island and tapers off

towards the island edge. The sand not only acts as a reservoir but also as a recharge agent. The

water contained in the sand reservoir is effected by the sea tides. In the case of this very small

island where groundwater is intensely exploited, the quality of most of the groundwater varies

from brackish to saline with electrical conductivity (EC) varying between 800 – 4000 µS (Fig.

2).

Fig. 1 Pari Island lithology

Sand

Corals

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Fig. 2 Map showing distribution of groundwater electrical conductivity values.

From the above discourse it was decided that in order to increase the capacity of the

groundwater to meet the demand for freshwater, an ASRRG (Artificial Storage Recharge and

Recovery of Groundwater) system constructed aimed to increase the capacity of the fresh

groundwater by the injection of rainwater. ASR-wells (aquifer storage and recovery) are able to

infiltrate water into an aquifer and to later extract water according to the demand[8,9,10]

. The

ASRRG system was installed at the western end of Pari Island where land was available. To

observe and monitor the increase of the volume of freshwater, a geoelectrical survey and direct

EC measurements were carried out. These observations were executed before and after the

installation of the ASRRG system.

METHODOLOGY

Research Area

Pari Island is one of the Seribu Islands group, Jakarta DKI (Capital Special Area), its

geographical coordinates being 5˚52’50” - 5˚54’50” S.Lat and 106˚34’00” - 106˚38’00” W.Long

(Fig. 3). The area of Pari Island is 41.32 Ha. Its topography being flat 0 – 3 meters a.s.l., with a

white sandy beach with mangrove vegetation covering the northern and southern part of the

5

island[1]

. The beautiful nature of Pari Island attracts many tourists, which on the other hand, puts

a stress on the groundwater resources, being the sole local source of freshwater.

Source: Google Maps

Fig. 3 Location of the research area on Pari island

Methods

The Ghyben-Herzberg relation between fresh and saline groundwater in which fresh

groundwater is lighter than saline groundwater is underlying principle of this research method.

ASRRG is a technology of collecting rainwater and through recharge wells store it to increase

the (fresh) groundwater capacity and to restore the lowered groundwater level. This technology

has been applied on the mainland to overcome a number of problems such as drought and

flooding. Design and management of artificial recharge systems involves geological,

geochemical, hydrological, biological, and engineering aspects[11]

.

The main problem facing small islands is the limited availability of fresh groundwater

and saltwater intrusion/coning as a result of overexploitation and sea-level rise. This technology

aims to prevent sea-level rise from negatively affecting groundwater quality and to inject

Research Area

JAKARTA BAY

INDONESIA

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rainwater as much as possible into the ground. The rainwater was collected from the run-off from

the roof of the office of the Pari Island Technical Executing Unit (UPT) of the LKSDMO and

through a gutter piped to the perforated infiltration wells. A system of 12 infiltration wells are

connected by a perforated pipe which is called the gallery pipe. The components of the ASRRG

include the rainwater gutter, gallery pipes and recharge wells. The material used for the rainwater

gutter, and the various types are PVC tubular of 10, 8 and 4 inch size (Fig. 4).

Fig. 4 ASRRG Construction

Before installation of the ASRRG (Indonesian:SIMBAT ) system, a survey of the EC of

groundwater at the ASRRG site has to be carried out to act as a base-line input. The ASRRG

system comprises 13 recharge wells of which one is located outside the well configuration, while

1he remaining 12 wells are connected by a network of gallery pipes. A protective wall surrounds

the ASRRG wells to prevent tidal influence. Observations on the increase in the volume of the

freshwater resources are carried out before and after the installation of the ASRRG. The

observations comprise the change in electrical conductivity and geoelectric investigation. The

change in EC is conducted at every ASRRG well while the geoelectric investigation is carried

out along traverses around the ASRRG system (Figure 6).

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Fig. 5 Lay-out of the ASRRG system on Pari Island

Fig. 6 Geoelectric survey lines at ASRRG constructrion on Pari island

The equipment used for the geoelectric survey was a SuperSting R8/1P with 56 swift

electrodes. In geoelectric surveying, everything depends on the desired depth of penetration, the

shallower the target the more detailed the results (data). Because Pari Island is relatively flat and

groundwater level is at shallow depth (< 1 meter), geoelectric measurements were carried out to

a maximum depth of 10 meters. The specific resistivity of various lithologies is shown on Fig. 7.

Recharge Well, diameter 10 Inch,

Depth 3meters

D epth 3 meter Gallery Pipe, diameter 4 Inchi

Rainwater Drain from Gutter

to Recharge Wells Well 1

2 3 4 5 6 7 8 9

10 11 12

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Fig. 7 Value range of specific resistivities[12]

RESULTS AND DISCUSSION

The freshwater lense on Pari Island is of very limited extend (volume) because the strong

influence of the seawater resulting in the fact that groundwater in thie area is mostly brackish to

salty. ASRRG technology employed on this island is expected to increase the size/volume of the

freshwater lens by lowering the salinity of the groundwater as the ASRRG principle is to catch

rainwater and inject it as much as possible through recharge wells. According to the Ghyben-

Herzberg principle, as the specific gravity of freshwater is smaller than of saltwater, it follows

that if sufficient large amounts of freshwater, except for some lesser influence of sea tides, the

freshwater will lower the freshwater-saltwater boundary resulting in an increased volume of

fresh groundwater. Before ASRRG rainwater runoff dispersed in all directions and mixed with

seawater to become brackish or even salty groundwater. As explained earlier, changes in

groundwater salinity on Pari Island are monitored by measuring electrical conductivity in the

ASRRG wells, and by geoelectrical surveys around the ASRRG installation area.

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Pore Volume

Increase in pore volume will result in an increase in rainwater infiltrating in the ground,

which will increase the amount of freshwater as expressed in the increased thickness of the

freshwater containing part of the aquifer. As such the design of the ASRRG system aims to

increase the pore volume to maximize infiltration of freshwater. Pore volume increase is

achieved in the ASRRG technology through the volume of the recharge wells and the volume of

the gallery pipes.

Pore volume increase through ASRRG = Recharge well volume + Gallery pipes volume (1)

Calculation of this volume is achieved using the pipe volume calculation , as follows:

Pipe Volume = ¼ x 𝜋x diameter2 x pipe length (2)

Well Pipe = Diameter 10 inch = 25,4 cm, Length 3 m = 300 cm, number of wells: 12

Gallery Pipe = Diameter 4 inch = 10,16 cm, Length 10 m = 1000 cm, number of pipes: 11

Well Volume = ¼ x 3.14 x 25,42 x 300

= ¼ x 3.14 x 645,16 x 300

= 151935,18 cm3

= 0,1519 m3

Volume of 12 Wells = 12 x 0,1519

= 1,8228 m3

Volume gallery pipes = ¼ x 3.14 x 10,162 x 1000

= ¼ x 3,14 x 103,2256 x 1000

= 81032,096 cm3

= 0,81 m3

For 11 gallery pipes = 11 x 0,81

= 0,89 m3

Total ASRRG Volume = Well Volume + Gallery Volume

= 2.71 m3

The result will be that the increase in pore volume for recharging will be 2.17 m3, while earlier

calculation based on a sand porosity of 30% (before ASRRG) resulted in a pore volume of 0.81

m3[13,14]

. During infiltration testing, the water injected in the ground was directly absorbed,

indicating a very high injection rate in this area. This implies that no matter the volume, water

injected in the ground will be quickly absorbed.

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Electric Conductivity

Measurement of electric conductivity in ASRRG wells is carried out using an EC meter.

Instantaneous measurements were carried out without considering sea tidal movements or depth

of the well. All ASRRG wells have about the same depth which is 0.5 m below sea-level.

Monitoring has been carried out with the same equipment and method except that it was done at

varying times, namely in September 2015 and March 2016. The results of the measurements are

shown in Table 1.

Table 1 Results of electric conductivity measurements (from Analyses 2016)

No Wells EC Before

ASRRG

Sept 2015

(µS/cm)

EC After

ASRRG

March 2016

(µS/cm)

Percentage

Decline

(%)

1. Well 1 47800 1700 96.44

2. Well 2 Not measured 4400 -

3. Well 3 Not measured 6780 -

4. Well 4 41600 14000 66.34

5. Well 5 22000 8380 61.90

6. Well 6 56300 5080 90.97

7. Well 7 41200 4640 88.73

8. Well 8 51650 3550 93.12

9. Well 9 53600 32700 38.99

10. Well 10 Not measured 24900 -

11. Well 11 50500 11300 77.62

12. Well 12 Not measured 18000 -

Geolectric Investigations

Geoelectric measurements were carried out before and after the construction of the

ASRRG system, the measurements before construction were carried out in September 2015,

while those carried out after construction were executed in February 2016. The measurements

carried out at different times were along the same traverses and using similar methods. The

results show the change in thickness/volume of the freshwater body/lens. Saline water is

indicated by low resistivity values, while freshwater is indicated by high resistivity values.

Measurements were conducted along 4 traverses before the ASRRG system was set up, and

along 5 traverses after the system was set up, with a target depth of 12 meters.

Traverse 5

This traverse is located along the northern side of the ASRRG and Pari Island. The

traverse has a length of 120 meters with a penetration/target depth of 12 meters, as shown on

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Fig. 8. The figure shows 2 resistivity sections along the traverse, the upper one showing the

results of the pre-ASRRG survey (2015), the lower one the results of the post-ASRRG survey

(2016). The pre-ASRRG geoelectric measurements shows that the groundwater along the

traverse is brackish to saline with resistivity values between 0 – 1 ohm-meter. In the pre-ASRRG

geoelectric measurement show no indication of fresh water. On the other hand the results of the

measurements after the installation of the ASRRG observation points where resistivity values

indicate the prescence of freshwater are more widespread. Thus the change in the fresh

groundwater interval between the measurements in 2015 and 2006 is 1 meters.

Fig. 8 Geoelectric Traverse 5

Traverse 6

This traverse cuts Pari Island in a North-South direction, the length of the traverse being

100 meters (Fig 9). The 2015 resistivity measurements show values of less than 2 ohm-meter.

This value indicates that the lithology comprises brackish to saline groundwater filled fine sand

to mud. The measurements of 2016, however, indicate an increase in resistivity values indicating

that the groundwater in the area along the traverse is fresh to brackish. The thickness of the

freshwater interval varies from 0.5 – 1 meter till a depth of 1 meter. This indicates a positive

change in groundwater quality which initially was dominated by brackish to saline into fresh to

brackish groundwater.

Breckish water

Brackish-Saline water

Saline water

Saline water

Fresh water Breckish water

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Fig. 9 Geoelectric Traverse 6

Traverse 7

This traverse runs parallel to Traverse 6, which cut across the Pari Island with a length of

105 meters (Fig. 10). Measurements carried out in 2015 indicated that for the larger part the

groundwater in the area along the traverse was saline, and only on a number of locations, for

examplr at location 50, indications of freshwater were encountered and probably brackish. On

the other hand the measurements in 2016 indicated significant changes where from surface to a

depth of 2 meters fresh groundwater was indicated with resistivity values between 8 – 20 ohm-

meters.

Fig. 10 Geoelectric Traverse 7

Brackish water

Saline water

Saline water

Saline water

Saline water

Saline water Saline water

Brackish water

Brackish water Freshwater

Brackish water

Brackish water

Brackish water

Fresh water Fresh water

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Traverse 8

This traverse is located at the southern side of the ASRRG paralleling the southern

coastline. The length of the traverse is 180 meters with a target depth of 13 meters. The

measurements of 2015 and 2016 indicate a salinity change of the groundwater. During the 2015

survey the area along the traverse indicated that the groundwater was predominantly brackish to

saline, while the measurements of 2016 indicate the existence of a fresh groundwater lense

indicated by the red color on Fig. 11, with a thickness of up to 2 meters.

Fig. 11 Geoelectric Traverse 8

DISCUSSION

Lowering of groundwater salinity at Pari Island has been achieved through the injection

of fresh water in the ground after the application of ASRRG. Installation of an ASRRG set-up

was by construction of a number of recharge wells connected by gallery pipes. The objective of

the construction of an ASRRG system was to increase the recharging pore volume, to be able to

inject freshwater in as large as possible volumes in the ground. Perforations were made with a

diameter of 0.2 mm at a distance of 10 cm were pade in the recharge wells and the gallery pipes.

The perforations aim to enable the water in the recharge wells and gallery pipes to force out the

saltwater in the area of the ASRRG and its vicinity.

The success rate of the installation of the ASRRG system can be concluded through the

monitoring of the salinity of the groundwater before and after the employment of the ASRRG

system. The monitoring is to be carried out by measuring the electrical conductivity (EC) and

through Geolectric measurements. EC measurements indicate a reduction of salinity of 77%

indicating a significant decrease in salinity. The geoelectric measurements performed before and

Saline water

Breckish water

Breckish water

Fresh water Fresh water

Breckish water

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after the installation of an ASRRG system indicate a reduction of resistivity values indicating the

formation of a freshwater lens where there was none before. The thickness of the freshwater lens

varies with a maximum thickness of 2.5 meters. The amount of rainfall injected and captured in

the ASRRG is considered sufficient to lower the salinity.

The installation of an ASRRG system is considered effective in lowering groundwater

salinity in small islands. By increasing pore volume from 30% to 60% is capable of decreasing

salinity by 77% and this is indicated by the formation of a freshwater lens, where before there

was none, with a thickness of 2.5 meters. The increase in pore volume is comparable to the

decrease in salinity. Increasing pore volume for groundwater recharge can be achieved either by

increasing diameter size of the ASRRG or through the extensification of the ASRRG network.

CONCLUSIONS

The conclusions which can be drawn from this research are the following

1. The greater the pore size/volume for recharging the groundwater the greater the thickness

of the freshwater lens.

2. The percentage increase of pore size/volume for recharging is in linear comparison to the

reduction percentage of decrease in groundwater salinity in small islands.

3. Increasing recharging pore size can be achieved by modification of the construction of

the ASRRG system.

4. The construction of ASRRG with a network system proves to be very effective to

increase the groundwater resource potential in small islands.

ACKNOWLEDGMENTS

The authors would like to thank Lembaga Indonesian Institute of Sciences (LIPI), the

Geotechnology Research Center (Puslit Geoteknologi) LIPI, UPT LPKSDMO Pulau Pari LIPI,

and the people of Pulau Pari for their support to the research on small islands. The researchers

also want to express their gratitude to Mr. Yayat Sudrajat, Nyanjang, and Sutarman for their

assistance in the field an studio. They also want to thank Dr. Fred Hehuwat and Mr. Hendra

Bakti for the discussions on the hydrology of small islands.

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